Field of the Invention
The present invention relates to a wire electric discharge machine configured to perform speed commanding in consideration of the difference between a circular-arc portion of a desired shape and its corresponding portion of a machining path.
Description of the Related Art
In wire electric discharge machining, a desired shape is usually created with reference to a drawing. A movement path of the center of a wire, based on this desired shape in consideration of the discharge gap, wire diameter, etc., is referred to as “machining path” herein. The discharge gap, wire diameter, etc., as added conditions, are collectively referred to as “offset”.
FIGS. 1A and 1B show how convex and concave circular-arc portions of a workpiece are subjected to wire electric discharge machining, respectively. In these drawings, reference numerals 3 and 4 denote a desired shape (full line) and a machining path (dotted line), respectively. The offset is the difference between the machining path 4 and the desired shape 3.
In actual machining, a computerized numerical controller attached to a wire electric discharge machine controls the wire electric discharge machine so that a wire electrode (wire) relatively moves along the machining path, thereby finishing the workpiece into the desired shape. A straight portion of the desired shape extends parallel to its corresponding portion of the machining path. As shown in FIG. 1A, therefore, the workpiece is machined for an arbitrary distance L as the wire moves for the same distance L. Thus, the moving distance of the wire is equal to the machining length of the workpiece.
In the case of a convex circular-arc portion to be machined, however, a distance a′ for the desired shape 3 is shorter than a length a of its corresponding portion of the machining path 4, as shown in FIG. 1A. Consequently, a length a′ of the workpiece to be machined as the wire moves for the distance a along the machining path 4 is shorter than the length a, so that the discharge density becomes so high that the workpiece is over-machined or machined deeper than in the machining of the straight portion.
In the case of a concave circular-arc portion to be machined, in contrast, a machining distance b′ for the desired shape 3 is longer than a length b of its corresponding portion of the machining path 4, as shown in FIG. 1B. Consequently, a length b′ of the workpiece to be machined as the wire moves for the distance b along the machining path 4 is longer than the length b, so that the discharge density becomes so low that under-machining occurs more easily than in the machining of the straight portion. Thus, over-machining and under-machining occur in the cases of machining the convex and concave circular-arc portions, respectively, so that the shape accuracy is inevitably reduced.
The following is a description of some examples of prior art techniques proposed to solve these problems.
A: Technique for Limiting Speed at Circular-Arc Portion
A-1: Japanese Patent Application Laid-Open No. 2001-162446 discloses a control method for wire electric discharge machining. If a machining path for the wire electric discharge machining includes an arcuate path portion, according to this method, machining conditions for the arcuate path portion are calculated based on conditions for electric discharge machining in a normal straight-line machining range and a function of an arc radius as a variable, in order to improve shape sagging in the arcuate path portion. In addition, a high-speed machining condition is applied to machining in the normal straight-line machining range, while machining along an arcuate machining path is controlled under a medium-speed machining condition.
In the technique described above, the straight portion is machined under the high-speed condition, and the circular-arc portion is machined under the medium-speed condition. In the case of the convex circular-arc portion, however, the machining distance for the desired shape is shorter than a length of its corresponding portion of the machining path, so that a distance a′ for which a workpiece is machined as a wire moves for an arbitrary distance is shorter than the distance a. Therefore, the discharge density becomes so high that the workpiece is over-machined or machined deeper than in the machining of the straight portion. To prevent this over-machining, the convex circular-arc portion should be machined more quickly than the straight portion. According to this technique, however, the convex circular-arc portion is machined under the medium-speed condition or more slowly than the straight portion. As a result, the shape accuracy of the circular-arc portion is inevitably reduced.
A-2: Japanese Patent Application Laid-Open No. 6-126536 discloses a wire electric discharge machine comprising calculation means configured to calculate a command speed by adding a speed change amount obtained based on an error voltage to a previous calculated value, calculate a speed target value based on a function of a programmed arc radius and an electrode offset amount in circular-arc machining, and set the speed target value as the command speed if the command speed is not less than the speed target value.
In the technique described above, command speeds for circular-arc portions are calculated by the same method as those for straight portions, and the upper limit value of the speed is calculated based on the function of the programmed arc radius and the electrode offset amount. In the case of circular-arc portions, as described with reference to FIGS. 1A and 1B, however, the desired shape and the machining path are different in arc radius. If the command speed for the circular-arc portion is calculated in the same method as for the straight portion, therefore, the desired shape cannot always be obtained. Further, some coefficients are needed to calculate the upper limit value of the speed for the circular-arc portion, and a method for obtaining these coefficients is not specified. Thus, experimental acquisition of these coefficients requires much time and labor.
According to the techniques A-1 and A-2, the shape accuracy of the circular-arc portion is improved by limiting the speed for the circular-arc portion.
B: Technique for Limiting Machining Speed at Corner Portion by Machining Amount
B-1: Japanese Patent No. 5241850 (corresponding to WO2010/050014) discloses a wire electric discharge machine in which a machining speed for each locus of the center of a wire at a corner portion is limited by a corner speed limiting unit, depending on an estimated machining amount, and a machining condition for each machining step is compensated according to the arc radius of a programmed locus at a portion corresponding to the corner portion.
In the technique described above, a machining speed for the corner portion is controlled according to a machining amount, but, in a case where a shape accuracy could not be secured by only control of the machining speed, compensation is made using a compensation value for each arc radius stored in a memory. However, a method for obtaining the compensation value for each arc radius is not specified, so that experimental acquisition of compensation values corresponding to all arc radii requires much time and labor.
B-2: Japanese Patent Application Laid-Open No. 2004-148472 discloses a wire electric discharge machining method in which four points of modification are set at a corner portion and an appropriate feed speed is obtained by multiplying a set feed speed by the ratio between a removal distance for each section and a straight portion. A constant feed system is used for wire feed in this technique.
In the technique described above, the four points of modification are set and the appropriate feed speed is obtained by multiplying a set feed speed by the ratio between a removal distance for each section and a straight portion. However, it is actually difficult to set these points of modification and constant-speed feed is a precondition, so that the scope of application of this technique is very narrow.
B-3: Japanese Patent No. 5077433 (corresponding to WO2010/001472) discloses a wire electric discharge machining apparatus configured so that a machining speed for a corner portion is calculated by multiplying “machining volume for each predetermined unit distance at corner portion”/“machining volume for each predetermined unit distance at straight portion” by an average speed for the straight portion.
According to the technique described above, the “machining volume for each predetermined unit distance” can be estimated to some degree from a machining program in the case of rough machining. In the case of finish machining, however, the machining allowances for various parts considerably vary depending on the degree of achievement of the rough machining, so that the “machining volume for each predetermined unit distance” cannot be estimated. Thus, this technique is not applicable to the finish machining.
According to the techniques B-1 to B-3, the shape accuracy of the circular-arc portion is improved by controlling the machining speed according to the machining amount.